During the street reconstruction/reconfiguration era (1998 to 2008) In downtown Houston we added concrete unit paver crosswalks at many intersections. The colors and patterns were selected to support an identity to the location and correlate with the various districts: i.e. the Theater District; the Skyline District, etc. They clearly define the area of the crosswalk but their installation and maintenance is challenging. This article will highlight several of the situations you want to avoid if you install similar crosswalks.

First you will want to know the design so click on this image from our plans to see the cross-section detail. Also keep in mind that this is in Houston Texas where there are frequent rains but no snow or ice. I also want to acknowledge that these crosswalks are in the areas with the most pedistrian traffic and that is where there are the most buses. Buses have many tires and are in the curb lane. Read on to see why tires and the gutter line are part of the challenge.

The first challenge is that manholes and valve boxes will inevitably be in the paver area and very seldom can a saw cut be made in a circular line to provide the very tight fit with the metal frame so the sand-filled gap is very small. The problem of paver failure at these locations evolved in this sequence:

the rain and tires racing by washed the sand out of the wider gaps;

the sand below the paver becomes saturated and pushes into the void space so the paver settles;

the fallen paver area around the frame becomes a basin that holds water that further penetrates the supporting sand layer;

soon a wider area is “pumping” (a phrase meaing that the space in the sand particles is completely filled with water that serves as a lubricant and the particles flow over each other);

the top edges of the pavers chip away as the paver sides are no longer parallel and the top edges push against each other and spall away and;

the area around the manhole frame or valve box crumbles away in a matter of 1-2 years.

That same sequence occurs at the ends of the crosswalk where the saw-cut pavers meet the radius of the curving gutter as it turns the corner. That radius is much larger than at a manhole but the pavers inevitably settle at the gutter’s edge, water seeps in, tires race by over the saturated sand, and the process begins.

Lessons Learned:

1. The solution is to prevent the wide gaps by pouring a concrete “collar” around the frame. The outside edges of the frame parallel the pattern of the pavers so there are not irregular cuts. If you are installing the pavers for the first time or completely relaying the crosswalk then make the collar the size to be an exact multiple of the paver width and length so there are no cuts that allow wider gaps. Ideally begin laying the pavers from that square collar and then all the cuts will be at the sides and ends of the crosswalk.

2. The concrete-collar solution does not work at the ends of the crosswalk unless you made the end square so the paver cuts would be straight. We do not have any built like that but likely there would be fewer failures at that location. The truth is the failures at the ends is predominantly where the gutter is lower than the nearby inlet and water ponds in very shallow “bird baths” over the pavers. The traffic in the curb lane on the right side is predominantly busses with more tires and more weights splashing through the “bird baths”.

3. Be sure there is some slope from the paver section’s edge to the gutter and on to the inlet. Otherwise that will be one of the first locations that begins to fail.

There are other articles in this blog about the use of pavers and the lessons we learned. I hope you will leave comments about your experiences so others can benefit from your experience.

Please do not take this as bashing crosswalks made of pavers. We have many crosswalks that have not had notable failures but all need maintenance. I only want to pass on lessons we learned and to provide some insight into what your maintenance budgets should include.

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This Blog…

This blog is a collection of articles about things I, Tom Davis, or others have built in downtown Houston Texas, what we learned and issues we faced. The hope is you will pick up ideas and tips from that process to allow your urban revitalization to benefit from these experiences.

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